1. Field of the Invention
The present invention relates to a reflected light intensity ratio measuring device for measuring a reflected light intensity ratio of a substrate to be treated to a standard substrate, a measuring device for measuring a light energy absorption ratio of the substrate to be treated to the standard substrate, and a heat treatment apparatus incorporating the measuring device.
2. Description of the Background Art
Conventionally, a lamp annealer employing a halogen lamp has been typically used in the step of activating ions in a semiconductor wafer after ion implantation. Such a lamp annealer carries out the activation of ions in the semiconductor wafer by heating (or annealing) the semiconductor wafer to a temperature of, for example, about 1000° C. to about 1100° C. Such a heat treatment apparatus utilizes the energy of light emitted from the halogen lamp to raise the temperature of a substrate at a rate of about hundreds of degrees per second.
In recent years, with the increasing degree of integration of semiconductor devices, it has been desired to provide a shallower junction as the gate length decreases. It has turned out, however, that even the execution of the process of activating ions in a semiconductor wafer by the use of the above-mentioned lamp annealer which raises the temperature of the semiconductor wafer at a rate of about hundreds of degrees per second produces a phenomenon in which the ions of boron, phosphorus and the like implanted in the semiconductor wafer are diffused deeply by heat. The occurrence of such a phenomenon causes the depth of the junction to exceed a required level, giving rise to an apprehension about a hindrance to good device formation.
To solve the problem, there has been proposed a technique for exposing the surface of a semiconductor wafer to a flash of light by using a xenon flash lamp and the like to raise the temperature of only the surface of the semiconductor wafer implanted with ions in an extremely short time (several milliseconds or less). The xenon flash lamp has a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamp is shorter than that of light emitted from the conventional halogen lamp, and approximately coincides with a basic absorption band of a silicon semiconductor wafer. It is therefore possible to rapidly raise the temperature of the semiconductor wafer, with a small amount of light transmitted through the semiconductor wafer, when the semiconductor wafer is exposed to a flash of light emitted from the xenon flash lamp. Also, it has turned out that a flash of light emitted in an extremely short time of several milliseconds or less can achieve a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time by using the xenon flash lamp allows the execution of only the ion activation without deeply diffusing the ions.
A heat treatment apparatus employing such a xenon flash lamp, which exposes the wafer surface to light having ultrahigh energy in an extremely short time, rapidly raises the temperature of the surface of the wafer, to cause the abrupt expansion of only the surface of the wafer. It has also turned out that, if the energy from the xenon flash lamp is excessive, the abrupt expansion of only the wafer surface results in the occurrence of slip on the wafer surface or the occurrence of a crack in the wafer in the worst case. On the other hand, the less light energy for the exposure fails to activate the ions. It is therefore important to optimize the range of the light energy emitted from the xenon flash lamp.
In general, when a flash lamp which emits light in an extremely short time is used, it is impossible to effect feedback control of a lamp output based on the measurement result of the temperature of the semiconductor wafer. For this reason, the following technique is used. A bare wafer which is not patterned is implanted with ions and is actually exposed to light. After the heat treatment, characteristics (e.g., sheet resistance and the like) of the wafer are measured. The light energy provided from the xenon flash lamp is adjusted based on the result of the measurement.
However, most wafers to be actually treated are patterned and are hence different in light absorbing characteristic from unpatterned bare wafers. When exposed to light of the same amount of energy, the patterned wafers tend to absorb more light energy than the bare wafers. For this reason, there has been a problem such that the wafers to be actually treated absorb more light energy to result in cracks in the wafers although the energy for exposure of the bare wafers is optimized. To prevent this, a correction for each wafer to be treated must be made to the proper value of the energy for exposure of the bare wafers.
To solve such a problem, Japanese Patent Application Laid-Open No. 2005-39213 discloses a technique to be described below. Prior to flash heating, this technique includes: measuring the reflection intensities of a standard wafer having a known reflectivity, an unpatterned wafer (e.g., a bare wafer) and a semiconductor wafer to be actually treated to calculate the value of light energy absorbed by the unpatterned wafer and the value of light energy absorbed by the wafer to be treated; calculating the light energy absorption ratio of the wafer to be treated to the unpatterned wafer based on the calculated light energy values; and calculating a proper value of energy to be applied to the wafer to be treated from the calculated light energy absorption ratio and a proper value of light energy to be applied to the unpattemed wafer.
In short, the technique disclosed in Japanese Patent Application Laid-Open No. 2005-39213 uses the reflection intensity of the unpattemed wafer as a reference value to calculate the proper value of energy to be applied to the wafer to be treated based on a result of relative comparison between the reference value and the reflection intensity of the wafer to be treated. A device for performing a computation based on such a relative comparison with the reference value involves the need to measure the reflection intensity of the unpattemed wafer at regular time intervals to store the measured result again and again (which is known as calibration), for example, in order to respond to factors of variations with time, such as aged deterioration, of a light source for reflection intensity measurement.
The calibration process, however, necessitates a stop of the actual operation (the process of the wafer to be treated) of the heat treatment apparatus to conduct a measurement of the reflection intensity of the unpattemed wafer. This, of course, lowers the rate of operation of the heat treatment apparatus by the amount of the stop time. Further, the management and storage of wafers for calibration have been burdensome because it is desirable to continue using the same wafers for calibration.